Synthesis and secretion of Ig/total cell protein were studied in the cells of 6 selected human lymphoid lines in continuous suspension culture for greater than 18 months. Incorporation of the radiolabeled amino acid leucine followed by immunoprecipitation using a sandwich antibody technique was used. All lines synthesized and secreted both H and L chain antigens: as reported earlier micron and k were the predominant Ig antigens detected and each was present in all tested lines. Some lines secreted up to four different H chains (micron, gamma, alpha, sigma) in uncloned cultures, as well as in experiments with cloned subcultures. Preliminary data using agar diffusion suggests the possibility of hybrid Ig molecules. The secretion rates between different lines varied to a greater degree than total Ig or total protein synthesis. No quantitative interrelationship was found between micron and sigma chain production. The rate of sigma chain secretion appears comparable to other H chain antigens. Igs synthesized by tonsillar cells in short-term culture with pokeweed mitogen demonstrated Gm allotype antigens corresponding to those found in the serum of the donor.